Radio frequency (RF) equipment uses a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. Typically, filtering structures are used to maintain proper communication in frequency bands assigned to a particular band. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, dielectric and coaxial cavity resonator filters are often used for filtering electromagnetic energy in certain frequency bands, such as those used for cellular and PCS communications.
The resonant frequency of certain resonators, known as low power factor concentric conductor transmission line resonators, partly depends on the projected length of the inner conductor, which changes in response to temperature variations. In stabilizing the resonant frequency of the line against changes due to variations in temperature, temperature-induced changes in this length are balanced or counteracted by changes in other dimensions. These counteracting dimensional changes have been achieved in various ways. For example, if a copper plate is used to form a cup-shaped wall over the top of a center conductor in the resonator cavity, the change in temperature causes the distance between the free end of the center conductor and the copper plate to change. This change affects resonant frequency and can be used to stabilize the resonator over temperature.
Another such temperature compensation scheme, using this general approach, employs a stabilizer strip fixed to a top plate (or cover) over the resonator cavity and facing the end of the center conductor. Securing the stabilizer strip to the top plate is labor intensive and can cause the resonator to become mistuned. Moreover, because the stabilizer strip is secured to the top plate, which is a relatively fixed point, differences in the lengths of resonator taps in adjacent resonators produce different distances between the heads of the resonator taps and the top plate. These differences are often on the order of millimeters, resulting in significantly different compensation requirements for different resonators. With these different requirements, using a single stabilizer strip design for the resonators can produce poor temperature compensation. To improve temperature compensation, this approach often involves redesigning the stabilizer strip dimensions for each cavity, increasing the complexity and cost of manufacture.